Control systems for electro-pneumatic converters are known. Usually a 4 to 20 milliamp electrical signal is used to actuate a solenoid-like motor. The 4 to 20 milliamp electrical signal causes a proportionate displacement in the spring-loaded core of the solenoidlike motor which displacement is used to control the restriction of an associated pneumatic valve producing a pressure change proportional to the motion of the core. An example of such a device may be found in U.S. Pat. No. 3,334,642 issued Aug. 8, 1967 to P. G. Borthwick.
There are certain disadvantages to the pneumatic servo assemblies of such electro-pneumatic control systems. Firstly, they are unable to hold positions on loss of power. Should power be removed from the coil, the core moves back to a position where it is in equilibrium with its associated spring. This causes the pneumatic output signal to go off scale, resulting in the movement of control devices actuated by the electropneumatic system to either the fully-opened or fullyclosed positions which may be catastrophic under certain circumstances. Secondly, such pneumatic servo assemblies are vibration sensitive. Since the cores are suspended from springs which act as range and zero limiters, vibration of the core causes a variation in the pneumatic output signal. Also, there usually is no feedback signal of the pneumatic output signal to the input of the control systems.